Implantable medical devices (IMDs) provide a variety of monitoring, diagnostic, and therapy functions. For example, implantable pulse generators (IPGs) provide low power cardiac pacing and implantable cardioverter defibrillators (ICDs) provide high power defibrillation therapies (often in addition to pacing). These devices often monitor cardiac performance and provide targeted therapies based upon the data collected.
The performance of any given IMD is limited by the type of data collected. For example, most IMDs readily and accurately monitor heart rate. This parameter, while basic, provides invaluable information for cardiac devices. In many instances, this parameter alone provides sufficient information for the complete and proper operation of the device. IPGs may be set so as to deliver pacing whenever a patient's heart rate falls below a predetermined value (bradycardia). Often, the desired minimum value will be based on a secondary parameter, such as an activity level as sensed by an accelerometer. Thus, a desired target heart rate is set for a patient based upon a sensed activity level. If the patient's heart rate falls below this level, then the IPG provides a pacing therapy. As these values change and the patient's heart responds differently, the IPG preferably delivers therapy only when needed.
In the high power context, heart rate is also a core parameter. Ventricular tachycardia (VT) and ventricular fibrillation (VF) are potentially dangerous arrhythmias that generally correlate to heart rate. For example, VT would generally be classified for a rate of 150 to 250 beats per minute (bpm), while VF would be classified for rhythms greater than 250 bpm. Of course, during exercise, a patient's heart rate may fall within this prespecified VT range despite that rate being normal for the level of activity. Conversely, VF may occur at a lower rate and still be problematic. Thus, there is a desire to provide additional discrimination when detecting and categorizing VT/VF.
As VT/VF may be life threatening, discrimination protocols are set to err on the side of caution and provide therapy. False positives are problematic in that the high energy therapy may be physically uncomfortable and even painful to the patient. Furthermore, the therapy is often delivered without warning to the patient thereby taking them by surprise, even though the therapy is appropriately applied to a genuine arrhythmia. Therefore it is desirable to lower the frequency of both necessary and unnecessary shocks.
Many actual ventricular (or atrial) tachyarrhythmias spontaneously terminate after a very short duration. Since the hemodynamic status of the patient is unknown to the device at the time of therapy, most devices are programmed to deliver therapy as quickly as possible following detection. The time to therapy is generally limited by the time required to detect and confirm the arrhythmia as well as the time for the main defibrillation capacitors to charge. If hemodynamic parameter describing the stability of the arrhythmia were provided to the device, then an option would be available to delay therapy so that the arrhythmia might terminate spontaneously thereby avoiding the need to shock.